Many Union booklets are rare and highly catalogued if complete.
Incomplete booklets can be exploded to display the contents such as the
interleaving, postage rates etc. as well as examples of the panes.
There are a handful of cheaper booklets such as the 6d “Razor” booklet,
so called due to its size being similar to a packet of old fashioned
razor blades and the last two Union booklets. They would make
interesting additions to a collection. Also look out for complete panes
used and booklet stamps or panes used on cover. They can be
particularly elusive.